Rotating stall and surge are important machine phenomena which can lead to dangerous machine damage in rotating machines such as a compressor as a result of a plurality of reasons. First, rotor vibrations resulting from the stream line degradation can damage labyrinths between stages. In addition, flow reversals can cause continuous increase in temperature at impeller vane entrances (and a corresponding decrease in discharge head capability), resulting in surge cycling. Also, pressure variations between intake and discharge ends of the compressor can cause rapid changes in axial thrust, thereby risking damage to the thrust bearing. Furthermore, abrupt load and speed changes may occur thereby detrimentally affecting the impellers and other internal compressor and driver components.
Heretofore, rotating stall and surge was viewed as an aerodynamic problem and aerodynamically the rotating stall and surge has been investigated extensively. This research has led to the following common industry definitions of local stall, stage stall, stall zone, surge and rotating stall. Local stall is a flow separation or reversal in either an impeller or diffuser. Stage stall is when a local stall increases to the point where one in a series of centrifugal impellers (and associated inlet and discharge stationary components) experiences reverse flow in part of its cross-sectional flow area. The overall flow is still in a forward, pressurizing direction. A stall zone is any cross-sectional area of an impeller or diffuser which undergoes a flow perturbation and which manifests symptoms of a stall in a centrifugal compressor. Surge is defined as periodic flow oscillations and pressure swings. If these oscillations include flow reversals it is a deep surge. The rate of gas flow is not enough to adequately fill the spaces between consecutive guide vanes. Surge propagates axially. Rotating stall, also called propagating stall, consists of large stall zones covering several blades and passages. Rotating stall propagates circumferentially at some fraction of rotor speed. The number of stall zones and the propagating rates vary considerably.
Heretofore, the solution to the phenomena of rotating stall and surge has been to monitor and alter the pressure flow within the compressor by altering the discharge pressure rate and/or the inlet pressure rate via valves. In addition, the monitoring of pressure and temperature has been used as a signature for detecting stalls. Furthermore, mechanical structures have been added to the compressor in the form of geometrically altered blades and vanes and free wheeling rotors. Therefore, even though aerodynamically the rotating stall phenomena has been investigated extensively, the rotor dynamic implications of this phenomena has received little attention. Thus, a need exists for a better understanding of rotating stall in centrifugal compressors from vibration diagnostics and rotor dynamic standpoints. In addition, a need exists for an apparatus and method which provides a vibrational diagnostic and rotor dynamic solution to the problem and not merely interacting to the aerodynamic results of rotating stall and surge conditions.
The following prior art reflects the state of the art of which applicant is aware and is included herewith to discharge applicant's acknowledged duty to disclose relevant prior art. It is stipulated, however, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.
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